1. Field of the Invention
The present invention relates to a piezoelectric thin-film resonator.
2. Description of the Related Art
In piezoelectric thin-film resonators, in order to acoustically isolate a vibrating portion, in which a piezoelectric thin film is disposed between a pair of excitation electrodes facing each other, from a substrate, a structure is used in which a thin-film member (membrane) is partially suspended above the substrate with an air-gap layer therebetween.
For example, as schematically shown in FIGS. 1A and 1B, a piezoelectric thin-film resonator 10 has a structure in which a dielectric film 12, a lower electrode 14, a piezoelectric thin film 16, and an upper electrode 18 are disposed on a substrate 11. An air-gap layer 13 (refer to FIG. 1A) is formed between the substrate 11 and the dielectric film 12. A portion in which the electrodes 14 and 18 overlap each other in the stacking direction and which includes the dielectric film 12, the lower electrode 14, the piezoelectric thin film 16, and the upper electrode 18, i.e., a vibrating portion 19, has a structure that is suspended above the substrate 11. The air-gap layer is formed by removing, in the final stage, a sacrificial layer 17 (refer to FIG. 1B) disposed between the substrate 11 and the dielectric film 12.
A thin-film member including the dielectric film 12, the lower electrode 14, the piezoelectric thin film 16, and the upper electrode 18 has a supported portion that is supported by the substrate and a suspended portion that is suspended above the substrate. In general, the suspended portion includes a middle section extending parallel to the substrate and a sloping section that is connected to the supported portion and the middle section, the sloping section extending obliquely with respect to the substrate. A part in which the sloping section is connected to the supported portion or the middle section has a bent shape in cross section (for example, refer to Japanese Unexamined Patent Application Publication No. 61-218214).
In the piezoelectric thin-film resonator having the structure that is bent in cross section as described above, breaking of the piezoelectric thin film and disconnection of the electrodes easily occur.
That is, in a resonator using a fundamental wave vibration mode, a membrane revealed after a sacrificial layer is removed includes a piezoelectric thin film and a pair of electrodes sandwiching the piezoelectric thin film. In such a case, since the strength of the membrane is low, the membrane may be broken, microcracking may occur, or the vibrating film may be deflected after the sacrificial layer is removed.
Strength can be provided to the membrane by forming a dielectric film on the upper or lower side of the piezoelectric thin film. However, if the dielectric film is formed such that sufficient strength is provided, the resonant bandwidth is decreased, thus degrading the resonant characteristic. When such a resonator is used, the bandwidth of a filter is also decreased, and it is not possible to fabricate a desired filter. Furthermore, when the dielectric film is formed, the stress of the dielectric film is added to the stress of the piezoelectric thin film, which may result in a further decrease in the strength of the membrane or deflection of the vibrating portion.
Furthermore, in the dicing step in which the individual elements are separated, breaking of the diaphragm easily occurs because the strength of the membrane may be insufficient. Moreover, during the mounting operation, since the vibrating portion is exposed, a load is likely to be applied to the vibrating portion, and breaking defects may easily occur in the element.
Consequently, the bending angle at the sloping section, i.e., the inclination angle relative to the substrate (slope angle), is preferably set as small as possible.
However, when the distance between the substrate and the middle section of the suspended portion is constant, it is necessary to increase the area of the sloping section of the suspended portion as the slope angle decreases, and the size of the element must be increased.
Consequently, in practice, the slope angle is at most limited to about 15 degrees, and it is not possible to completely eliminate the breaking of the piezoelectric thin film, disconnection of the electrodes, and the like.